We describe the design and optimization of a wide-field near-infrared camera and spectrograph (NICAS) for Mt Abu 1.2
m, f/13 Cassegrain telescope of Physical Research Laboratory. The principal science goals include photometric mapping
of star forming regions and medium resolution spectroscopy of Young Stellar Objects, evolved stars and transient
sources. The design goals are to achieve seeing-limited angular resolution in an un-vignetted field of view of ~ 8'x8' with
0.5" per pixel (of 18.5 μm) on a HgCdTe 1024×1024 infrared array, requiring a two-fold Cassegrain focal reduction. In
addition to the imaging, the instrument is required to have spectroscopic capability with a resolving power of 103 in the
0.85 - 2.5 μm region, needing a dispersion of 1 nm per pixel. Finally, since our telescope has a moderate aperture, the
throughput losses need to be minimized. The specifications are achieved by an optical design using 9 singlet lenses.
Only those lens materials are chosen for which measured values are available for refractive indices at 77 K (detector
operating temperature), changes of indices with temperature, and thermal coefficients of expansion. The design is
optimized to give sharpest images at 77 K. The optical path is folded by 90° after collimation by a fold-mirror and re-imaged
on the detector. The fold-mirror is replaced by a diffraction grating for spectrograph mode. In order to minimize
the reflection losses, all the lenses will be anti-reflection coated for the full operating wavelength range. Details of the
design are presented.